PETM: Methane and Global Warming
56 Mio years ago, a global warming occurred once before, driven by greenhouse gases in abundance like today! This event is called the Paleocene Eocene Thermal Maximum (PETM).
Due to an initial warming, maybe caused by exceeding volcanic eruptions, clathrate hydrates in ocean bed became unstable, releasing previously frozen methane into the atmosphere. Methane is a potent greenhouse gas, hundred times more powerful than CO2. The extra warming through increased levels of methane started a strong positive feedback!
The global mean temperature on Earth rised from averaged 18°C up to 23°C, causing major changes in marine and terrestrial ecosystems.
PETM is indicated by three proxies:
Indicators of PETM in the oceans (IPCC, 2007).
Global rise in temperature decreased O18/O16 ratio in oceans through enhanced evaporation of the heavier isotope. There is a temperature-sensitive fractioning of oxygene isotopes via differential evaporation from oceans to atmosphere.
The release of methane decreased C13/C12 ratio. Carbon isotope C13 is sparse in Methane, as this greenhouse gas on Earth mainly originates from microbes and life which discriminates enzymatically against it. Hence a release of methane leads to a diminished C13/C12 isotope ratio.
Still in the oceans a part of released large amounts of methane was oxidated to CO2. CO2 and water interact chemically to carbonic acid/ CO2 + H2O -> H2CO3, H2CO3+H2O->H3O (+) + HCO3(-)/ dissolving carbonates (CO3) on the seefloor.
Over the last years we have witnessed an extraordinary rise of methane in the Arctic. Methane is a greenhouse gas much more powerful than CO2! Source: AMEG
Global warming maybe has started methane-greenhouse feedback mechanism again! But this time it happens in the Arctic! Compared with other regions of planet Earth the Arctic is warming disproportionately due to ice-albedo effect. Temperatures are rising and highly reflective Arctic sea-ice is melting and replaced by dark open water absorbing sunlight very well. Additional warming speeds up melting of sea-ice etc. pp. causing a positive feedback. And so in the warming Arctic ocean methane clathrate hydrates into seabed became unstable again releasing more and more methane into the atmosphere. Do we already have passed a vital threshold? It would seem that a fatal runaway greenhouse effect is possible and thereafter a planet Earth, maybe Cretaceous style!
Jens Christian Heuer